When Shemp first attempts to enter the ring to spar with The Chopper, he becomes entangled in the ropes. The Chopper can be clearly seen standing just outside the ring with his fully gloved hands on his hips. The scene quickly cuts to a closeup of The Chopper, whose now-bare right hand is pulling up the glove on his left hand.
After Shemp spills moth balls on the floor, he grabs an ax, but in the next shot, the ax disappears.
An earlier shot with Shemp sneaking past cardboard boxes is repeated after Moose accidentally knocks out Big Mike.
When Shemp thinks he is shot, he clutches his hand with his chest, but in the next shot, his hand is on his side.
The magazine and the dumbbell in Chopper's hands swap places in another shot.
When Moe slaps Larry while saying "What's good for a fractured skull?" Larry's response is "I don't know, either." This was ad-libbed by Moe and Larry's reaction is real.
When Moose gets knocked out by the two bricks, you can see a large hole in the crate tarp big enough for Moe to reach for Moose's head, that wasn't there when Moose first sat down.
When Shemp knocks out the bad guys with the ax, you can hear Cy Schindell, who plays henchman Moose, yelling "Now, Shemp!" as he hits the floor. Shemp apparently took a pause and waited for Schindell to hit the floor before knocking out the lead bad guy as he was supposed to knock them both out at once.
You can see a moving shadow behind the paint can when a hole is shot through it.